Piezoelectric transducer

ABSTRACT

Various piezoelectric transducers are provided. In one form, a polymer film or thin ceramic piezoelectric transducer is formed of various piezoelectric (active) and dielectric (inactive) layers in which the piezoelectric effect may be attenuated locally at any given point on the surface of the transducer, by use of printed circuit patterns, preferably made by photolithography. This provides a practical realization of a distributed piezoelectric transducer with a bi-dimensional polarization profile that varies smoothly over the surface of the piezoelectric structure. Flexibility of the fabrication procedure provides a way to optimize the design of a distributed piezoelectric transducer for applications such as active vibration control. In another form, a segmented piezoelectric transducer includes a set of active elements such as piezofilms, electronics and flexible printed circuits and connected with external electronic circuitry. The segmented piezoelectric transducer uses a modulation scheme to combine the individual sensor outputs. Modal coordinates may be extracted from a test structure on which the segmented piezoelectric transducer is attached. As well, it is possible to use a distributed piezoelectric transducer to actuate a structure by using a time-varying polarization profile.

[0001] This application claims the benefit of U.S. provisionalapplication Ser. No. 60/210,119 filed Jun. 7, 2000 entitled DistributedPiezoelectric Laminate for Modal Sensing/Actuating of Thin StructuralComponents.

FIELD OF THE INVENTION

[0002] The present invention relates generally to piezoelectric sensorsand actuators and, more particularly, to piezoelectric laminates,segmented piezoelectric film sensors, and the application thereof tomodal analysis.

DESCRIPTION OF THE PRIOR ART

[0003] During the past decade, distributed piezoelectric sensors andactuators (collectively, transducers) have been used increasingly in thefield of active vibration sensing and control. The basic idea is to bonda piezoelectric lamina onto a large surface of a thin vibratingstructure. By collecting the piezoelectric charge induced by surfacestrains, one can estimate the vibrational state of the structure.Reversely or conversely, by applying an appropriate control voltage tothe transducer, it is possible to excite (actuate) the structure andthus, for example, to generate an artificial damping effect or monitorstructural behavior. Among the advantages of this approach are the lightweight of the piezoelectric transducer and the possibility to embed thedevice in thin, flexible structures that are prone to vibrate. Use ofthis technology can be found from the aerospace industry tomanufacturers of high-end consumer goods such as “smart skis” (i.e. skisthat incorporate such transducer technology).

[0004] For this technology to be efficient, it is necessary for thepolarization of the piezoelectric lamina to vary in a well-definedmanner over the surface of the transducer. Since the piezoelectricsensitivity of a laminate is normally uniform, a weighting function mustbe applied to the transducer with the help of some controlled physicalprocess.

[0005] It has been demonstrated that by measuring or inducing strains ina thin bending structure (such as a beam or plate) by means of a shapedpiezoelectric lamina bonded on its surface, it is possible to sense oractuate directly a given modal contribution of a transverse motion,provided that the boundary conditions satisfy certain requirements thatresult in the orthogonality of these modes.

[0006] In this method, it can be shown that the local piezoelectricsensitivity of the transducer must follow an appropriate spatial sensingdistribution (or weighting) function Λ(α₁, α₂) in order to interact witha unique mode. It should be noted that the use of the two parameters α₁and α₂ do not limit the domain of A to a plane. Instead, α₁ and α₂ arebi-dimensional curvilinear coordinates that can define any point on theneutral surface of a three-dimensional shell. Since the piezoelectricsensitivity of a laminate is usually uniform, a weighting function mustbe implemented with the help of a controlled physical process.Sometimes, the operation of imposing a given polarization profile on aplanar piezoelectric transducer is also called “shading”. There areseveral available procedures for shading.

[0007] In one procedure, the longitudinal piezoelectric constants of thepiezoelectric material can be adjusted to a value proportional to aweight function Λ(α₁, α₂), by means of a manufacturing method such asrepoling, doping or the dosage of a two-phase composite. It should benoted however that these techniques are costly and difficult toimplement in practice.

[0008] In another procedure, a given area of a piezoelectric transduceris made effective by the presence of a pair of electrodes which caneither collect the charges generated piezoelectrically (sensor mode) orimpose an electric potential between them (actuator mode). Thus, anelementary type of spatial weighting is obtained by limiting the areasprovided with electrodes. In this case, the weighting function takesonly two values: Λ=1 inside the shape boundaries, or Λ=0. In practice, aphotolithographic process can implement this profile.

[0009] For a beam, it is sufficient to implement a one-dimensionalweight function. In this case, the appropriate variation ofpiezoelectric sensitivity can be obtained by varying the width of theelectrode down the length of the beam since there is no deflection alongthis direction. To impose the correct sign of the weight (spatialsensing distribution) function, areas are defined where the polarizationof the piezoelectric material should be accordingly positive ornegative, either by bonding the laminate “face up” or “face down”, or byinverting its contacting electrodes.

[0010] In the more general case of a structure whose deflection variesalong two directions, a bi-dimensional spatial sensing distributionfunction can be approximated by a lattice of small electrodes that areturned either “on” or “off”. However the task remains to connectelectrically the individual active electrodes and to impose the correctfunction sign. In practice, this is only feasible if the “on” electrodeshappen to be grouped in a few contiguous domains.

[0011] In spite of the elegance of their concept, modal sensors andactuators have an important limitation. A manufacturing process allowingcontrol the weight function by repoling, doping, or dosing a two-phasecomposite is usually not available at the level of the applicationengineer. Furthermore, these steps would be very costly to implement. Asa consequence modal sensors have not been applied to structuralelements, such as plates and shells, unless their weight function couldbe reduced to a one-dimensional function or a product of such functionsby separation of variables.

[0012] This difficulty and other factors have promoted the use of anapproximated version of modal sensors, called segmented piezoelectricsensors. In this design the distributed effect of a piezoelectriclaminate is replaced by an array of size-limited, discrete piezoelectricsensors, each of which are measured separately, and the outputs of whichare being sampled, multiplied by discrete weight factors (calculated bythe method of modal filtering), and then added. The main advantage ofthis method is that it shifts the operation of fixing the weight factorsfrom the manufacturing process to an electronic operation, making itmuch more flexible. Thus, except for the number of channels, it is notmore difficult to build such a system for a variety of shells and platesrather than a beam.

[0013] However, segmented piezoelectric sensors also have theirshortcomings. For one, segmented piezoelectric sensors are only able tomodel a finite number of modes (the more transducers in the array, thehigher this number). If unmodelled modal contributions (residual modes)are present, they constitute a source of noise. For another, segmentedpiezoelectric sensors are much less compact than modal sensors. Eachchannel requires a full measurement chain including coaxial cable, lowcurrent or charge sensitive amplifier and analog to digital converter.Furthermore, a digital signal processing board is required to carry outthe computations to estimate the modal coordinates. By contrast, a modalsensor simply needs a unique coaxial cable and a low current or chargesensitive amplifier. For this reason it is more difficult to embedsegmented piezoelectric sensors as elements of an intelligent structure.Finally, because of the analog to digital conversions involved withsegmented piezoelectric sensors, it is difficult to include segmentedtransducers in sensing-actuating applications, such as thefrequency-stabilizing element in a resonator. On the other hand, the useof a modal sensor in a resonating circuit is straightforward and mayopen the door to applications where the frequency of such a system couldbe used to monitor physical parameters, like the temperature of thestructure or a variation of pressure exercised on it.

[0014] Because of the limitations imposed by these technologies, it isvery difficult to obtain a polarization profile whose shape varies infunction of two geometric dimensions. Thus, in order to apply thisactive control scheme to structures whose deflections vary along twoindependent coordinates, such as vibrating plates and shells, one of thefollowing simplifying methods is normally used.

[0015] One such method is that the behavior of a bending plate may beapproximated with the one of a bending beam, so that the electrodeshaping method described above may be used.

[0016] Another such method is that an arbitrary, bi-dimensionalpolarization profile may be approximated with a discrete pattern.Namely, one may juxtapose a finite set of segmented piezoelectricsensors/actuators whose individual contributions are weighted and addedup electronically.

[0017] A further such method is that a polarization profile can beroughly approximated by applying a “binary” weighting function, i.e. onein which contiguous areas are multiplied by values of either 0 or 1.This can be realized by bi-dimensional shaping of the measuringelectrodes.

[0018] However, in each of these simplified embodiments, the efficiencyof the control scheme is eventually affected. Possible problems mayrange from perturbations by residual modes to instabilities viaspillover. The use of segmented sensors/actuators may also posepractical problems if many channels are required because of extra weightadded to the test structure by the connect cables. As a consequence,there is a need for a new generation of distributed piezoelectrictransducers in which arbitrary polarization profiles could beimplemented in a versatile and practical manner.

[0019] There is also a need for a piezoelectric transducer that canefficiently implement a weighting function/distribution.

[0020] There is further a need for a new generation of segmentedpiezoelectric transducers.

SUMMARY OF THE INVENTION

[0021] In one form (referred to herein as Form A), the present inventionis a piezoelectric transducer. Particularly, in one form the presentinvention is a multi-layered piezoelectrically active and inactivelamina structure having selective metalizations that implement abi-dimensional weighting function of piezoelectric sensitivity.

[0022] When the lamina structure is attached to a thin structuralelement, the piezoelectric transducer may be used as a modal sensorand/or actuator.

[0023] The present invention provides at least the following advantagesover known solutions to the problem of sensing or actuating transversemodes of vibration in thin structures, which may be applied to variousapplications too numerous to list: (1) compared to shaped piezoelectricmodal sensors and actuators, it is straightforward to implement withthis design any arbitrary two-dimensional weight function for structuresuch as plates and shells. Also, the problem of forcing the correct signof the weight function is solved in the invention by collectingpiezoelectrically induced charges on both faces/surfaces of activelamina of the piezoelectric laminate in a first configuration, and ontwo individual laminas of the piezoelectric laminate in a secondconfiguration; (2) modal sensors whose weight functions have beenbuilt-in by modulating the piezoelectric sensitivity of the material bya manufacturing means, such as repoling, are very costly and often thetechnology is not readily available, if at all, to engineers, while thetechnologies required by the present invention in order to implement theweight function (e.g. photolithography or screen printing) are widelyavailable and economical; (3) segmented piezoelectric sensors require afull measurement chain (i.e. a transmitting cable, an impedanceconverter such as a charge amplifier, and a sampling circuit convertingthe analog signal into digital form) for each channel. This priorapproach becomes very expensive, and thus not practical for modalfiltering because of the high number of channels. As well, perturbationsby residual modes are then more likely to occur. By contrast, a veryhigh density of weighted “channels” can be implemented with the presentinvention at no special cost, the limitation being set by the resolutionof the available photolithographic or screen-printing process. Also,prior segmented piezoelectric sensors and actuators require an analog todigital conversion (digital to analog, respectively) and dedicateddigital signal processing boards. For the present invention, all that isneeded is a low-current or charge sensitive amplifier in the sensingmode and a sine voltage generator in the actuating mode for the presentpiezoelectric laminate.

[0024] In another form (herein referred to as Form B), the presentinvention is a segmented piezoelectric film sensor. The segmentedpiezoelectric film sensor includes a set of active elements mounted ontoa base and connected with external circuitry. The set of active elementsincludes piezofilms/sensors, individual electronics for each segment,and a flexible printed circuit. The external circuitry provides amodulation scheme to combine individual sensor outputs and to extractmodal coordinates from a test structure on which the segmentedpiezoelectric sensor is mounted.

[0025] The invention has advantages over prior segmented piezoelectricsensors and to modal filtering using segmented piezoelectric sensors. Inparticular, compared to prior segmented piezoelectric sensors, thepresent invention simplifies the required hardware by replacing N-2measurement chains—each comprising a low current or charge sensitiveamplifier and a coaxial cable—with 2N digital modulation signals andelectric wires, and by eliminating the need for a digital signalprocessing board, the system output being a signal directly proportionalto the modal coordinate of interest. Also, the invention is always ableto provide an estimate of the modal coordinate in real-time, even if avery large number of piezosensors have to be taken into account.Finally, the use of a flexible printed circuit in the present inventionmakes it easier to connect the active components mounted on thestructure to the external electronic circuitry. As well, compared toshaped piezoelectric modal sensors, the system has the flexibility ofbeing able to modify its weighting function, either to improve theestimate of the modal coordinate of interest, or to monitor a differentmodal coordinate.

BRIEF DESCRIPTION OF THE DRAWING

[0026] The above-mentioned and other features and advantages of thisinvention, and the manner of attaining them, will become more apparentand the invention will be better understood by reference to thefollowing description of an embodiment of the invention taken inconjunction with the accompanying drawing comprised of a plurality offigures, wherein:

[0027]FIG. 1 is a perspective view of a piezoelectric lamina structurein accordance with the principles of Form A of the present invention;

[0028]FIG. 2 is a an exploded view of the individual laminas of thepiezoelectric lamina structure of FIG. 1;

[0029]FIG. 3A is a block diagram of an exemplary system utilizing thepiezoelectric lamina structure in a sensor mode for Form A of theinvention;

[0030]FIG. 3B is a block diagram of an exemplary system utilizing thepiezoelectric lamina structure in an actuator mode for Form A of theinvention;

[0031]FIG. 4 is a more detailed diagram of an exemplary system utilizingthe piezoelectric lamina structure of FIG. 3A;

[0032]FIG. 5A relates to Form A of the invention and is a top plan viewof an upper surface or face of a lamina of one embodiment of thepiezoelectric lamina structure of FIG. 1 having an exemplary electrodepattern thereon;

[0033]FIG. 5B relates to Form A of the invention and is a top plan viewof a lower surface or face of the lamina of FIG. 5A having an exemplaryelectrode pattern thereon (Form A);

[0034]FIG. 6A relates to Form A of the invention and is a top plan viewof an upper surface or face of a lamina of another embodiment of thepiezoelectric lamina structure of FIG. 1 having an exemplary electrodepattern thereon;

[0035]FIG. 6B relates to Form A of the invention and is a top plan viewof a lower surface or face of the lamina of FIG. 6A having an electrodethereon;

[0036]FIG. 7A relates to Form A of the invention and is a top plan viewof an upper surface or face of another lamina of the second embodimentof the piezoelectric lamina structure of FIG. 1 having an electrodethereon;

[0037]FIG. 7B relates to Form A of the invention and is a top plan viewof a lower surface or face of the lamina of FIG. 7A having an exemplaryelectrode pattern thereon;

[0038]FIG. 8 is a block diagram of a system utilizing a segmentedpiezoelectric sensor (Form B) in accordance with the principles of anaspect of the present invention;

[0039]FIG. 9 is a perspective view of a segmented piezoelectric sensor(Form B) in an exemplary system of the manner of FIG. 8;

[0040]FIG. 10A is representation of switch positions in accordance withan aspect of Form B of the present invention;

[0041]FIG. 10B is representation of switch positions in accordance withan aspect of Form B the present invention;

[0042]FIG. 11A is a graph of an exemplary piezoelectric signal from thesystem of FIG. 9;

[0043]FIG. 11B is a graph of an exemplary pulse modulation signal forthe system of FIG. 9;

[0044]FIG. 11C is a graph of the combination of the signals of FIGS. 11Aand 11B;

[0045]FIG. 12A is a frequency domain representation of a baseband signalof the system of FIG. 9;

[0046]FIG. 12B is a frequency domain representation of a modulatedsignal of the system of FIG. 9;

[0047]FIG. 13 is another switch representation for the system of FIG. 9having a pulse-width modulation scheme; and

[0048]FIG. 14 is a further switch representation for the system of FIG.9 having a differential pulse-width modulation scheme.

[0049] Corresponding reference characters indicate corresponding partsthroughout the several views.

DETAILED DESCRIPTION

[0050] While the invention is susceptible to various modifications andalternative forms, the specific embodiment(s) shown and/or describedherein is by way of example. It should thus be appreciated that there isno intent to limit the invention to the particular form disclosed, asthe intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the invention asdefined by the appended claims.

[0051] Referring now to FIG. 1, there is shown an embodiment of apiezoelectric device, generally designated 20, in accordance with theprinciples presented herein. The piezoelectric device 20 is functionalas a sensor and/or an actuator and will hereinafter be referred to as apiezoelectric transducer 20. In general, the piezoelectric transducer 20is a laminate composed of a plurality of layers or lamina as hereinafterdescribed.

[0052] The piezoelectric transducer 20 includes a middle lamina 22, afirst upper lamina 24, a first lower lamina 26, a second upper or toplamina 28, and a second lower or bottom lamina 30, all of which isprovided on a base or structural component 32. Referring additionally toFIG. 2, the middle lamina 22 is defined by a plate or sheet 22 c of apiezoelectric active material. The plate 22 c has an upper surface orface 22 a and a lower surface or face 22 b. In accordance with theprinciples of the present invention, the upper and lower surfaces, 22 aand 22 b, are metalized (i.e. layered with an electrically conductivematerial such as a metal). As described more fully below, the upper andlower metalized surfaces 22 a and 22 b are patterned or etchedselectively, preferably by using a photolithographic or screen printingprocess.

[0053] The first upper lamina 24 is defined by a thin layer 24 c of apiezoelectrically inactive material of preferably a constant thickness.The layer 24 c has an upper surface or face 24 a and a lower surface orface 24 b. Preferably, the lamina 24 is an adhesive such that the lowersurface 24 b is bonded onto the upper surface 22 a of the lamina 22.

[0054] The first lower lamina 26 is defined by a thin layer 26 c of apiezoelectrically inactive material of preferably constant thickness inlike manner to the lamina 24. The layer 26 c has an upper surface 26 aand a lower surface 26 b. Preferably, the lamina 26 is an adhesive suchthat the upper surface 26 a is bonded onto the lower surface 24 b of thelamina 24. Preferably, the lamina 26 and the lamina 24 havesubstantially identical dielectric constants.

[0055] The second upper or top lamina 28 is defined by a plate or sheet28 c of a piezoelectrically inactive material. The plate 28 c has anupper surface or face 28 a and a lower surface or face 28 b. The upperand lower surfaces 28 a and 28 b of the plate 28 c are metalized but notetched or patterned. The lower surface 28 a of the plate 28 c is bondedto the upper surface 24 a of the plate 24 c.

[0056] The second lower or bottom layer 30 is defined by a plate orsheet 30 c of a piezoelectrically inactive material. The plate 30 c hasan upper surface or face 30 a and a lower surface or face 30 b. Theupper and lower surfaces 30 a and 30 b of the plate 30 c are metalizedbut not etched or patterned. The upper surface 30 b of plate 30 c isbonded to the lower surface 26 b of the plate 26 c.

[0057] The laminate 20 composed of laminas 22, 24, 26, 28, and 30 isbonded onto the base 32. The base 32 is defined by a shell, beam, orother structural component. The plate 32 c has an upper surface or face32 a and a lower surface or face 32 b. The lower surface 30 b of thelamina 30 is bonded to the upper surface 32 a of the base 32.

[0058] In FIG. 3A there is depicted a block representation of apiezoelectric sensor system generally designated 40. The piezoelectricsystem 40 includes the piezoelectric transducer 20 in communication withan amplifier 42 such that the system 40 provides a sensing mode for thepiezoelectric transducer 20. The piezoelectric transducer 20 providessignals that are amplified by the amplifier 42. The amplifier 42provides an output signal SOUT that represents piezoelectric signalsfrom the piezoelectric transducer 20.

[0059] In FIG. 3B there is depicted a block representation of apiezoelectric actuator system generally designated 44. The piezoelectricsystem 44 includes the piezoelectric transducer 20 in communication witha voltage generator 46, preferably generating a sine wave of a givenvoltage, such that the system 44 provides an actuating mode for thepiezoelectric transducer 20. The sine voltage generator 46 provides itssignal to the piezoelectric transducer 20.

[0060] Referring now to FIG. 4, there is shown an exemplary embodimentof a more detailed piezoelectric transducer system 40 as set forth inFIG. 3A. The piezoelectric transducer 20 is electrically coupled to theamplifier 42 via a coaxial cable 50. More particularly, the metalizedlower surface 28 b of the lamina 28 is coupled via an electrical line 48b to the coaxial cable 50 while the upper surface 30 a of the lamina 30is coupled via an electrical line 48 a to the coaxial cable 50. Theamplifier 42 is preferably a charge sensitive (or low-current) amplifierthat provides a voltage output signal V_(OUT).

[0061] Referring to FIGS. 5A and 5B, and in accordance with an aspect ofform A of the present invention, the upper and lower metalized surfaces22 a (FIG. 5A) and 22 b (FIG. 5B) of the middle lamina 22 are formedsuch that a pattern of openings 52 are formed in each surface. The upperand lower metalized surfaces 22 a and 22 b thus form electrodes. Thepattern of openings 52 in each metalized surface 22 a and 22 b arepreferably formed via a photolithography or screen-printing process(patterning). The surface density of the metalized surface, afterundergoing patterning, defines the magnitude of a weight function in agiven area. By grounding the electrodes (i.e. the upper and lowermetalized surfaces 22 a and 22 b) a transverse polarization fieldgenerated inside the piezoelectric lamina 22 has no external effectwherever the surface is covered by metalization. Thus, themetalization/electrodes locally act as shields to the polarizationfield. An opening 52, however, allows the transverse polarization fieldto extend out of the piezoelectric lamina 22, through the dielectriclayers (laminas 24 and 26) by capacitive coupling and finally reachesthe inside metalization of the laminas 28 and 30.

[0062] The following conditions are imposed on the geometry of theetched patterns: if there is an opening 52 on either face/surface (22 a,22 b) of the piezoelectric lamina 22, the opposite area facing it mustbe shielded (see FIGS. 5A and 5B, and note the alignment of the cornersABCD). Since the electrodes (metalization) of the lower surface 28 b ofthe lamina 28 and the upper surface 30 a of the lamina 30 arecontinuous, they collect the contributions of the transversepolarization field passing through the openings 52 of the uppershield/metalization/electrode 22 a and the lowershield/metalization/electrode 22 b, respectively, where they areconverted into a total electric charge. It should be noted that adistinction must be made between the electrodes (metalizations) of thelower surface 28 b of the lamina 28 and the upper surface 30 a of thelamina 30: the sign of a given transverse polarization field appears tobe reversed if seen from the measuring electrode (28 b) of the lamina 28to the grounded shield or if it is measured from the electrode (30 a) ofthe lamina 30 to the grounded shield. Therefore, a given deformation ofthe piezoelectric lamina results in an accumulation of charges ofdifferent signs on each measuring electrode. The electric currents thatare formed subsequently on these electrodes can be collected andmeasured by the low-current or charge sensitive amplifier 42.

[0063] If the relative size of each opening 52 is small compared to thevariations of the transverse polarization field, i.e. if the field ispractically constant over the surface of each opening 52, then it ispossible to weigh the piezoelectric effect over a given area by fixingthe size of the openings 52 covering a particular area of the lamina 22.In short, this effect is practically the same or similar to a sensingdistribution function Λ(α₁, α₂) weighting the local sensitivity of thepiezoelectric lamina 22 (the sign of which is controlled by locating theshield openings 52 on the upper or lower faces 22 a, 22 b of thepiezoelectric lamina 22). This weighting function is discrete since thedensity of the openings 52 in the shield (metalization) is limited bythe resolution of the available technology (i.e. photolithography orscreen printing process) as well as by capacitive border effects takingplace in the dielectric laminas (laminas 24 and 26). However, thedensity of weighting operations per area is still much higher than whatcan be achieved with typical segmented piezoelectric sensors. Forpractical purposes, a modal filter implemented in this manner canreasonably be seen as a new type of modal sensor or actuator.

[0064] In this design, the openings 52 in the shields (i.e. themetalization on the surfaces 22 a and 22 b of the lamina 22) are notsupposed to touch each other, since it would result in the isolation ofmetalized areas which could not be grounded any more. This restrictionlimits the range of the weight function from: −1<Λ<1, to approximately:−0.5<Λ<0.5. Also, the capacitive coupling operation reduces the strengthof the polarization field by a factor: γ=1/(1+K), where κ=(ε_(r)¹δ₂)/ε_(r) ²δ₁), ε_(r) ¹, ε_(r) ² are the relative dielectric constantof the laminas 22, 24, and 26, and δ₁, δ₂ are the thickness of thelaminas 22, 24, and 26, respectively. Therefore, the local signal thatcould be measured with the piezoelectric lamina is multiplied here by aweighting function having values in the following range: −0.5γ<Λ<0.5γ.

[0065] The outside metalization (surface 28 a and surface 30 b) of thelaminas 28 and 30, respectively, should be grounded to provide a globalelectric shield for the piezoelectric laminate 20. Likewise, theelectric connections 48 a and 48 b (see FIG. 4) between the measuringelectrodes (surface 28 b and surface 30 a) of the laminas 28 and 30respectively, are preferably realized with short coaxial cables, inorder to protect their signal against electromagnetic interferences.

[0066] In one form, it is possible to utilize polymer films for thevarious laminas. The middle lamina 22 can be made out of a bi-metalizedpiezoelectric polyvinylidene fluoride (PVDF) film. Thin commercialtransfer tape may be used as dielectric laminas/adhesives 24 and 26,while a thin bi-metalized polyester film may be utilized for the outsidelaminas 28 and 30.

[0067] In accordance with another aspect or form of the presentinvention and referring to FIGS. 1 and 2, the piezoelectric transducer20 again has five (5) layers or lamina that may be used with the systemsof FIGS. 3A, 3B, and 4. However, in this embodiment, the central ormiddle lamina 22 is made from a thin, piezoelectrically inactiveadhesive. The first upper lamina 24 is from a plate of piezoelectricallyactive material that is bonded to the middle lamina 22. The first lowerlamina 26 is made from a preferably identical plate of piezoelectricallyactive material that is bonded to the middle lamina 22. In essence, thelamina 24 and the lamina 26 are effectively bonded together via themiddle lamina 22 of adhesive.

[0068] With reference to FIGS. 6A and 6B, the upper and lower surfaces,24 a and 24 b, of the lamina 24 are covered, such as by metalization, byelectrodes. The upper surface 24 a has a pattern of openings 52 and 54that are preferably formed via photolithography or screen-printing. Themetalization of the lower surface 24 b of the lamina 24 is coupled to asuitable ground. The upper surface 24 a is coupled via lead 48 b to thecoaxial cable 50 (reference FIG. 4) such that any signal collected viathe electrode or metalization is coupled to the amplifier 42.

[0069] Referring to FIGS. 7A and 7B, the upper and lower surfaces 26 aand 26 b of the lamina 26 are covered, such as by metalization, byelectrodes. The upper surface 26 a is coupled to a suitable ground. Thelower surface 26 b has a pattern of openings 52 and 54 that arepreferably formed via photolithography or screen-printing. The lowersurface 26 b is coupled via lead 48 a to the coaxial cable 50 (referenceFIG. 4) such that any signal collected via the electrode or metalizationis coupled to the amplifier 42.

[0070] The lamina 28 is made of a piezoelectrically inactive(dielectric) material that is bonded onto the lamina 24. The outersurface 28 a of the lamina 28 is provided with an electrode ormetalization that is coupled to a suitable ground. The lamina 30 is alsomade of a piezoelectrically inactive (dielectric) material that isbonded onto the lamina 26. The outer surface 30 b of the lamina 30 isprovided with an electrode or metalization that is coupled to a suitableground. The electrodes of the upper surface 28 a and the lower surface30 b act as shields for the high impedance sensor (piezoelectriclaminate 20) against external electromagnetic interference.

[0071] With respect to this embodiment, the frequency of the openings 52and/or 54 are varied to control the amount of charge per area collectedby the measuring electrodes 24 a and 26 b, and therefore to simulate theeffect of a spatial weighting distribution function modulating theoverall piezoelectric sensitivity of the piezoelectric laminate 20.However, unlike prior configurations, the openings 52 and/or 54 or“holes” in the electrodes 24 a and 26 b are used to inactivate the areasthey cover, since the outer electrodes of the laminas 24 and 26 cannotcollect a free piezoelectric charge at these locations. The upper layer24 a of the lamina 24 is only active over areas where the weightfunction is positive, whereas the lower layer 26 b of the lamina 26 isonly active over areas where this function is negative.

[0072] Now, provided that the two piezoelectric laminas 24 and 26 sharethe same poling orientation, the polarization imposed by the respectiveposition of ground and measuring electrodes ensures that the signalsgenerated on each layer have the proper sign. The overall weightedresponse is finally obtained by adding the respective charges at thenode or junction of the electrical leads 48 a and 48 b, and the coaxialcable 50. It should be appreciated that in this configuration, thepiezoelectric signals need not be coupled capacitively to transmit thesignal to the amplifier 42, so that the piezoelectric sensor 20 willhave a higher signal to noise ratio than the other embodiment.

[0073] Again, commercially available piezoelectric PVDF film and thintransfer tapes may be used to build the present piezoelectric laminate20. The piezoelectric laminate 20 along with the amplifier 42 provides apiezoelectric sensor. Conversely, the piezoelectric laminate 20 alongwith an actuator or sine voltage generator 46 (reference FIG. 3B)provides a piezoelectric actuator.

[0074] Referring to FIG. 8, there is depicted a block diagram of apiezoelectric sensor system for form B of the invention, generallydesignated 60, in accordance with an aspect of the present invention.The system 60 includes a segmented piezoelectric sensor having integralcircuitry 62 in accordance with the present principles. A signalgenerator 64 is coupled to the segmented piezoelectric sensor 62 andprovides configured signals or sets of modulated signals to thepiezoelectric sensor 62. An integrator 66 is coupled to thepiezoelectric sensor 62 that receives signals from the various segmentedpiezoelectric units of the segmented piezoelectric sensor 62. A filter68 is provided for the output of the integrator 66, the filter 68providing a piezoelectric output signals represented by block 70.

[0075] In particular, and referring to FIG. 9, there is depicted anexemplary system, generally designated 80, implementing the system 40 ofFIG. 8. The system 80 includes a segmented piezoelectric sensor made inaccordance with the principles presented below, generally designated 82.The segmented piezoelectric sensor 82 includes a polymeric thin film,substrate, or the like 96 that is bondable to a test structure 94. Thepolymeric film 96 may be a polyester film, conductive epoxy, or thelike. A plurality of piezoelectric sensors 100 are disposed on thepolymeric film 96 and spaced from each other (i.e. segmented). Aplurality of microelectronic circuits 102 are also mounted on thepolymeric film 96, the number of which preferably corresponds with thenumber of the plurality of piezoelectric sensors 100. Preferably, amicroelectronic circuit 102 is adjacent each one of the piezoelectricsensors 100. A patterned plurality of electrodes and conductive lines 98are formed on the polymeric film 96 preferably by deposition, but whichmay take the form of any printed circuit or the like. The plurality ofelectrodes and conductive lines 98 connect each piezoelectric sensor 100with an associated microelectronic circuit 102, communicate with anoutput, and communicate with an input.

[0076] The system 80 further includes a signal generator 84 that iscoupled to the pattern of conductive lines 98 such that the signalgenerator 84 is in communication with each microelectronic circuit102/piezoelectric sensor 100 pair via a conductor 86 at the input of theconductive lines 98. The conductor 86 is preferably a ribbon cable.Connected at the output of the conductive lines 98 are two conductorsthat are preferably coaxial cables 92 a and 92 b for collecting thecurrents generated on the surface electrodes of each piezoelectricsensor 100, such as that shown and described above, and processed by therespective microelectronic circuit 102. The conductors 92 a and 92 b arecoupled to a differential charge amplifier 88 that integrates thecollected piezoelectric currents. The differential charge amplifier 88is, in turn, coupled to a low-pass filter 90 to demodulate the outputsignal U_(O).

[0077] The signal generator 84 is operative to generate a set ofdistinct modulating signals for each microelectronic circuit 102 in themanner set forth below. The modulation scheme in conjunction with thesegmented piezoelectric sensor 82 combines the piezoelectric signals.While the modulation signals are generated externally via the signalgenerator 84, the modulation signals may be generated by circuitry/logicintegral with the segmented piezoelectric sensor 82. It is possible toprovide dedicated signal generators via one or several applicationspecific integrated circuits (ASICs) and to mount the components on thestructure. These signals may be generated with bi-stable multivibrators,programmable logic devices, a microprocessor board, or the like.

[0078] The charge sensitive amplifier(s) 88 integrate the currentsproduced by the piezoelectric sensors 100 at the nodes defined by thecables 92 a and 92 b, and convert them into two low impedance voltagesignals that are then subtracted from each other. The virtual grounds ofthese amplifiers also hold the voltage across the piezosensors constant,so that parasitic capacitors in the cable or in transistors (MOSFETs) donot affect the signal. The time integration performed by thesecomponents has the effect of canceling out the high frequency noiseoriginating from the modulation signals and coupled through theparasitic capacitors of the MOSFETs (see below).

[0079] Each microelectronic circuit 102 includes, operates, andfunctions in the manner set forth below. Referring to FIGS. 10A and 10B,there is shown two states, positions, or modes of an exemplaryembodiment of a microelectronic circuit 102 as coupled to apiezoelectric sensor 102. Specifically, pairs of analog switches S_(1k)and S_(2k) can be implemented so that the output of the kth sensingpiezoelement is either directed to the remote charge amplifier 88(position I, FIG. 10A) or grounded (position II, FIG. 10B). In thesecases, the ON/OFF signals m_(1k)(t) and m_(2k)(t) actuating the switchesS_(1k) and S_(2k) are in phase opposition so that m_(1k)(t)=m_(k)(t) andm_(2k)(t) and −m_(k)(t). By alternating the positions I and II, theoutput current i_(k)(t) (see FIG. 11A) can be modulated by the signalgenerator 84 into a measured current i_(k) ^(A)(t) (see FIG. 11C) with apulse-width T_(k) and a period T_(m) (see FIG. 11 B) or pulse-widthmodulation (pwm).

[0080] Each analog switch may be implemented with a solid-state device,such as a metal oxide semiconductor field effect transistor (MOSFET).Since these devices are very small, a pair can be surface-mounted orembedded in the vicinity of each piezofilm without interfering with thedynamics of the test structure 94 (see FIG. 9).

[0081] Referring to FIG. 12A, there is shown a representation in thefrequency domain of the output current I_(k)(f). Referring to FIG. 12B,there is shown a representation in the frequency domain of the measuredcurrent I _(k) ^(A)(f). The baseband signal (FIG. 12A) can be recoveredby utilizing the low-pass filter 90 to low-pass filter the measuredcurrent, provided that the sidebands do not overlap with the baseband.This is satisfied under the conditions that: i) the signal measured fromthe k^(th) piezoelement in the sensor is bandlimited, i.e. there is amaximum frequency f_(max) such that I_(k)(f)=0 if f>f_(max), and ii) themodulating frequency f_(m)=1I/T_(m) is at least twice as high as themaximum frequency: f_(m)≧2 f_(max). It should be understood that theamplitude of the demodulated current Demod[i_(k) ^(A)(t)] differs fromthe baseband signal by a scaling factor related to the pulse width andmodulation frequency. Specifically:

Demod[i _(k) ^(A)(t)]=(T _(k) /T _(m))i _(k) ^(A)(t).

[0082] Each piezoelectric sensor 100 output can be modulated in the samemanner, but with different pulse-widths T_(k), so that afterdemodulation each signal appeared as multiplied by a specific factorL_(k)=(T_(k)/T_(m)). It is thus possible to add together the modulatedcurrents at a common node A, demodulate their sum i^(A)(t) with alow-pass filter and then time integrate the resulting current andconvert it into a voltage with a charge amplifier 88 a as shown. Inpractice, however, it is more convenient to first pass the modulatedcurrent i^(A)(t) through a charge amplifier and then proceed with thedemodulation, as shown in the FIG. 13 (by linearity, the order of theseoperations makes no difference).

[0083] In this system, the output yields the voltage:${{U_{out}(t)} = {\left( {1/C_{f}} \right){\sum\limits_{k = 1}^{N}\quad {L_{k}{q_{k}(t)}}}}},$

[0084] where C_(f) is the feedback capacitor of the charge amplifier andq_(k) is the charge generated by each piezosensor 100.

[0085] If the factors L_(k) in the above equation are identical with thei^(th) line of a gain matrix of a modal filter, then the output voltagewould be proportional to the i^(th) modal coordinate of the system.However, in this scheme the factors L_(k) display all the same sign,since the ratio T_(k)/T_(m) is always positive. For modes of higherorder than one, though, there will always be a number of negativeentries. In order to remedy this problem, a differential configuration88 b as shown in FIG. 14 may be used.

[0086] In this circuit the currents are not grounded during the phase II(see FIGS. 10A and 10B), but instead are being summed at a second node Band are then directed to a second charge amplifier. By subtracting u^(B)from u^(A) with a differential amplifier, the output voltage yielded bythe system is the same as given in the above equation, where now:

L _(k)=(R ₂ /R ₁ C _(f))[2(T _(k) /T _(m))−1].

[0087] As indicated in the above expression for the range of thecoefficients, it is possible via the configuration of FIG. 14 to selecta negative value for I_(k):

−R ₂ /R ₁ C _(f) ≦L _(k) ≦R ₂ /R ₁ C _(f)

[0088] With an appropriate set of modulation signals m_(k), i.e. acorresponding set of values for the pulse-widths T_(k)'s, the voltageoutput is made proportional to a modal coordinate of the system. Anothermodal coordinate can be monitored by selecting another set of values forthe parameters T_(k).

[0089] While this invention has been described as having a preferreddesign and/or configuration, the present invention can be furthermodified within the spirit and scope of this disclosure. Thisapplication is therefore intended to cover any variations, uses, oradaptations of the invention using its general principles. Further, thisapplication is intended to cover such departures from the presentdisclosure as come within known or customary practice in the art towhich this invention pertains and which fall within the limits of theclaims.

What is claimed is:
 1. A piezoelectric device comprising: a plurality ofstacked lamina; a first one of the plurality of lamina made of apiezoelectrically active material and having a first electrode on afirst surface thereof and a second electrode on a second surfacethereof, one of said first and second electrodes having a plurality offirst openings defining a weighting function; a second lamina of apiezoelectrically inactive material disposed adjacent said first surfaceof said first one of the plurality of lamina; and a third lamina of apiezoelectrically inactive material disposed adjacent said secondsurface of said first one of the plurality of lamina.
 2. Thepiezoelectric device of claim 1, wherein said second lamina is bonded tosaid first surface of said first one of the plurality of lamina via afourth lamina of said plurality of laminas, said fourth lamina of apiezoelectrically inactive material, and said third lamina is bonded tosaid second surface of said first one of the plurality of lamina via afifth lamina of said plurality of laminas, said fifth lamina of apiezoelectrically inactive material.
 3. The piezoelectric device ofclaim 2, wherein said fourth and fifth laminas are adhesives.
 4. Thepiezoelectric device of claim 1, wherein the other of said first andsecond electrodes includes a plurality of second openings defining theweighting function in conjunction with said first openings.
 5. Thepiezoelectric device of claim 4, wherein said first and second openingsare formed via one of photolithography and screen-printing.
 6. Thepiezoelectric device of claim 4, wherein said plurality of laminacomprises five lamina, said piezoelectrically active lamina forming amiddle lamina, said first outer lamina of a piezoelectrically inactivematerial is bonded onto a first surface of said piezoelectrically activelamina via a first adhesive lamina, and said second outer lamina of apiezoelectrically inactive material is bonded onto a second surface ofsaid piezoelectrically active lamina via a second adhesive lamina. 7.The piezoelectric device of claim 6, wherein said first and secondadhesive laminas are piezoelectrically inactive.
 8. The piezoelectricdevice of claim 4, wherein said first outer lamina has an electrode onan upper surface thereof and an electrode on a lower surface thereof,and said second outer lamina has an electrode on an upper surfacethereof and an electrode on a lower surface thereof.
 9. A piezoelectricdevice comprising: a first lamina of a piezoelectrically inactivematerial; a second lamina of a piezoelectrically active materialdisposed on a first surface of said first lamina, said second laminahaving a first electrode on a first surface thereof, and a secondelectrode on a second surface thereof, one of said first and secondelectrodes having a plurality of first openings defining a weightingfunction; a third lamina of a piezoelectrically active material disposedon a second surface of said first lamina, said third lamina having afirst electrode on a first surface thereof, and a second electrode on asecond surface thereof, one of said first and second electrodes having aplurality of second openings defining said weighting function; a fourthlamina of a piezoelectrically inactive material disposed on said secondlamina; and a fifth lamina of a piezoelectrically inactive materialdisposed on said third lamina.
 10. The piezoelectric device of claim 9,wherein said fourth lamina includes an electrode on an outside surfacethereof, and said fifth lamina includes an electrode on an outsidesurface thereof.
 11. The piezoelectric device of claim 9, wherein saidpatterns of first and second openings are formed by one ofphotolithography and screen-printing.
 12. The piezoelectric device ofclaim 9, wherein said first openings of said second lamina are adjacenta surface of said first lamina, and said second openings of said thirdlamina are adjacent another surface of said first lamina.
 13. Asegmented piezoelectric device comprising: a signal generator; apolymeric film; a plurality of piezoelectric sensors disposed on saidpolymeric film; a plurality of microelectronic circuits disposed on saidpolymeric film, each microelectronic circuit in communication with apiezoelectric sensor; a plurality of conductors disposed on saidpolymeric film and coupling each microelectronic circuit with saidsignal generator and an output; an amplifier in communication with saidoutput; a low pass filter in communication with said amplifier; andwherein said signal generator provides an input signal to saidmicroelectronic circuits.
 14. The segmented piezoelectric device ofclaim 13, wherein said signal generator is operative to produce amodulated signal.
 15. The segmented piezoelectric device of claim 14,wherein said signal generator is operative to produce a modulate signalfor each microelectronic circuit.
 16. The segmented piezoelectric deviceof claim 14, wherein each microelectronic circuit comprises switchcircuitry operative to provide an output signal in conjunction with saidsignal generator.
 17. The segmented piezoelectric device of claim 14,wherein said signal generator is a sine voltage generator and is incommunication with said plurality of conductors via a ribbon cable. 18.The segmented piezoelectric device of claim 14, wherein said amplifieris a differential charge amplifier.
 19. The segmented piezoelectricdevice of claim 14, wherein said amplifier is coupled to said output viaa pair of coaxial cables.
 20. A method of modal filtering comprising:providing a segmented piezoelectric device having a plurality ofpiezoelectric sensors disposed on said polymeric film, a plurality ofmicroelectronic circuits disposed on said polymeric film, eachmicroelectronic circuit in communication with a piezoelectric sensor,and a plurality of conductors disposed on said polymeric film andcoupling each microelectronic circuit with said signal generator and anoutput; providing a modulated signal to each microelectronic circuit;collecting an output from each piezoelectric sensor in an amplifier; andfiltering an output of said amplifier.